An ultra-compact projection display, called a micro-projector, can provide an image larger than the hosting projecting device dimension. Such displays have the potential to overcome the display size limitation on mobile devices and can be used to display documents and images, for example. The displays can also help to overcome the input interface size limitation on mobile devices by displaying virtual input devices, such as keyboards.
One technology used for micro-projection is laser scanning, in which red, green and blue lasers are scanned across a surface to form a full color image.
It is known that an individual semi-conductor laser requires dynamic intensity stability control, which is usually performed by monitoring the laser output with a photo-diode and using a feedback control loop to control the laser intensity.
When micro-projection is used in a mobile device, the environmental conditions, such as temperature, will be subject to large changes. These changes increase the need for intensity control.
In a full color micro-projector, the laser sources are very close together and some techniques used for feedback control of a single laser may be ineffective due to cross-talk between the lasers. For example, one prior approach is to monitor the main beam reflection in the system, either from the housing or from the lens, to provide a feedback signal. Another approach is to measure leakage from a back mirror of the cavity. However, this is not a direct measure of laser power and reduces laser efficiency. Neither of these approaches will work when multiple lasers are used, because they do not provide feedback for the individual lasers or they result in increased overall size of the system.